1. Field of the Invention
This invention relates to antenna drive mechanisms and, in particular, to a drive mechanism for a satellite antenna dish which utilizes a free-floating output gear within a gear housing to reciprocally vary the directional position of the satellite dish.
2. Description of the Prior Art
Satellite dish antennas have become widely used in recent years to receive communications, particularly video signals, from satellites orbiting the earth. Generally, these antennas, or earth stations, collect microwave signals broadcast by communications satellites orbiting above the equator. The satellites orbit at the same speed the earth rotates so that their positions remain fixed along a similar orbit, which allows broadcasters to use them as relay stations for their programming. Since a plurality of these satellites appear to lie in an arc extending from one horizon to the other, a satellite antenna should be capable of being directionally positioned along this horizon-to-horizon arc.
Various drive means have been developed to alter the directional position of a satellite antenna dish. Although various systems have been developed in connection with tracking and research antennas to provide complete hemispherical coverage, it is generally established that drive systems associated with communication satellite antennas need only move the antenna in an arcuate path in order to provide proper reception. In addition, because modern antenna dishes are designed for individual home use, the drive system should be as simple as possible to minimize the cost and complexity of the system. However, both manual and motor driven systems are well known.
Most of the past known horizon-to-horizon drive systems for satellite antennas utilize some variation of a worm-gear drive. These systems are generally mounted directly to the support shaft of the antenna and utilize a rotatable worm to drive a gear connected to the antenna dish. Variations of this system include the use of a gear rack or partial gears in place of the gear which controls the movement of the antenna dish. Because of the interaction between the components of these drive systems, the worm and gear must be kept well lubricated to minimize friction and prevent malfunction. In addition, the worm and gear drives are subject to severe backlash which causes misalignment of the antenna. Since the satellites from which the antennas receive their signals are over 20,000 miles above the equator, a 1.degree. misalignment of the antenna can cause a disruption or interruption of the communication signal. Once backlash between the intermeshing components of the system causes misalignment, a service technician must readjust the antenna in order to provide accurate directional alignment of the system.
In order to reduce jamming and backlash between the cooperating components of known drive systems, a drive system has been developed which utilizes a threaded bow screw that cooperates with a gear housing. Generally, these known systems utilize a bow screw with a diameter of 5/8" or smaller which cooperates with an internally threaded drive gear mounted within a gear housing. The gear housing includes an input gear engagingly meshed with the internally threaded output gear. Both gears have their hubs or axles fixedly secured to the housing so that their relative positions are fixed. The bow screw extends through and cooperates with the output gear whereby rotation of the output gear causes linear movement of the bow screw. However, because of the fixed relationship between the bow screw, the output gear, and the input gear, the load exerted by the antenna dish tends to cause jamming between the drive gears and between the output gear and the bow screw. Again, due to this jamming, backlash may occur causing misalignment of the antenna. In addition, because of the small diameter of the bow screw and the loads exerted by movement of the antenna, extensive use of the drive system will cause the bow screw to warp thereby limiting movement of the antenna to approximately 15.degree. from its center position. Although this range of movement is adequate for most applications, as additional communication satellites are placed within the equatorial orbit, this limited range of movement will render such drive systems obsolete sine they are not capable of complete horizon-to-horizon movement.
Thus, a drive system which provides complete horizon-to-horizon movement of the antenna dish while eliminating backlash due to jamming of the interacting components is necessary in order to meet modern requirements.